The present invention relates to magnetic memory systems, and more particularly to a method and system for providing an element that employs a spin transfer effect in switching and that can be used in a magnetic memory such as magnetic random access memory (xe2x80x9cMRAMxe2x80x9d).
Magnetic memories are often used in storing data. One type of memory element currently of interest utilizes magnetoresistance of a magnetic element for storing data FIGS. 1A and 1B depict conventional magnetic elements 1 and 1xe2x80x2. The conventional magnetic element 1 is a spin valve 1 and includes a conventional antiferromagnetic layer 2, a conventional pinned layer 4, a conventional spacer layer 6 and a conventional free layer 8. The conventional pinned layer 4 and the conventional free layer 8 are ferromagnetic. The conventional spacer layer 6 is norunagnetic. The conventional spacer layer 6 is conductive. The antiferromagnetic layer 2 is used to fix, or pin, the magnetization of the pinned layer 4 in a particular direction. The magnetization of the free layer 8 is free to rotate, typically in response to an external magnetic field.
The conventional magnetic element 1xe2x80x2 is a spin tunneling junction. Portions of the conventional spin tunneling junction 1xe2x80x2 are analogous to the conventional spin valve 1. Thus, the conventional magnetic element 1xe2x80x2 includes an antiferromagnetic layer 2xe2x80x2, a conventional pinned layer 4xe2x80x2, an insulating barrier layer 6xe2x80x2 and a free layer 8xe2x80x2. The conventional barrier layer 6xe2x80x2 is thin enough for electrons to tunnel through in a conventional spin tunneling junction 1xe2x80x2.
Depending upon the orientations of the magnetizations of the free layer 8 or 8xe2x80x2 and the pinned layer 4 or 4xe2x80x2, respectively, the resistance of the conventional magnetic element 1 or 1xe2x80x2, respectively, changes. When the magnetizations of the free layer 8 and pinned layer 4 are parallel, the resistance of the conventional spin valve 1 is low. When the magnetizations of the free layer 8 and the pinned layer 4 are antiparallel, the resistance of the conventional spin valve 1 is high. Similarly, when the magnetizations of the free layer 8xe2x80x2 and pinned layer 4xe2x80x2 are parallel, the resistance of the conventional spin tunneling junction 1xe2x80x2 is low. When the magnetizations of the free layer 8xe2x80x2 and pinned layer 4xe2x80x2 are antiparallel, the resistance of the conventional spin tunneling junction 1xe2x80x2 is high.
In order to sense the resistance of the conventional magnetic element 1/1xe2x80x2, current is driven through the conventional magnetic element 1/1xe2x80x2. Current can be driven through the conventional magnetic element 1 in one of two configurations, current in plane (xe2x80x9cCIPxe2x80x9d) and current perpendicular to the plane (xe2x80x9cCPPxe2x80x9d). However, for the conventional spin tunneling junction 1xe2x80x2, current is driven in the CPP configuration. In the CIP configuration, current is driven parallel to the layers of the conventional spin valve 1. Thus, in the CIP configuration, current is driven from left to right or right to left as seen in FIG. 1A. In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 1/1xe2x80x2. Thus, in the CPP configuration, current is driven up or down as seen in FIG. 1A or 1B. The CPP configuration is used in MRAM having a conventional spin tunneling junction 1xe2x80x2 in a memory cell.
FIG. 2 depicts a conventional memory array 10 using conventional memory cells 20. Each conventional memory cell 20 includes a conventional magnetic element 1/1xe2x80x2, depicted as a resistor in FIG. 2. The conventional memory array 10 typically uses a spin tunneling junction 1xe2x80x2. The conventional array 10 is shown as including four conventional memory cells 20. Each memory cell 20 includes a conventional spin tunneling junction 1xe2x80x2 and a transistor 22. The memory cells 20 are coupled to reading/writing column selection 30 via bit lines 32 and 34 and to row selection 50 via word lines 52 and 54. Also depicted are write lines 60 and 62 which carry currents that generate external magnetic fields for the corresponding conventional memory cells 20 during writing. The reading/writing column selection 30 is coupled to write current source 42 and read current source 40 which are coupled to a voltage supply Vdd 48 via line 46.
In order to write to the conventional memory array 10, the write current Iw 42 is applied to the bit line 32 or 34 selected by the reading/writing column selection 30. The read current Ir 40 is not applied. Both word lines 52 and 54 are disabled. The transistors 22 in all memory cells are disabled. In addition, one of the write lines 60 and 62 selected carries a current used to write to the selected conventional memory cell 20. The combination of the current in the write line 60 or 62 and the current in the bit line 32 or 34 generates a magnetic field large enough to switch the direction of magnetization of the free layer 8xe2x80x2 and thus write to the desired conventional memory cell 20. Depending upon the data written to the conventional memory cell 20, the conventional magnetic tunneling junction 1xe2x80x2 will have a high resistance or a low resistance.
When reading from a conventional cell 20 in the conventional memory array 10, the read current Ir 40 is applied instead. The memory cell 20 selected to be read is determined by the row selection 50 and column selection 30. The output voltage is read at the output line 44.
Although the conventional magnetic memory 10 using the conventional spin tunneling junction 1xe2x80x2 can function, one of ordinary skill in the art will readily recognize that there are barriers to the use of the conventional magnetic element 1xe2x80x2 and the conventional magnetic memory 10 at higher memory cell densities. In particular, the conventional memory array 10 is written using an external magnetic field generated by currents driven through the bit line 32 or 34 and the write line 60 or 62. In other words, the magnetization of the free layer 8xe2x80x2 is switched by the external magnetic field generated by current driven through the bit line 32 or 34 and the write line 60 or 62. The magnetic field required to switch the magnetization of the free layer 8xe2x80x2, known as the switching field, is inversely proportional to the width of the conventional magnetic element 1xe2x80x2. As a result, the switching field increases for conventional memories having smaller magnetic elements 1xe2x80x2. Because the switching field is higher, the current required to be driven through the bit line 32 or 34 and particularly through the write line 60 or 62 increases dramatically for higher magnetic memory cell density. This large current can cause a host of problems in the conventional magnetic memory 10. For example, cross talk and power consumption would increase. In addition, the driving circuits required to drive the current that generates the switching field at the desired memory cell 20 would also increase in area and complexity. Furthermore, the conventional write currents have to be large enough to switch a magnetic memory cell but not so large that the neighboring cells are inadvertently switched. This upper limit on the write current amplitude can lead to reliability issues because the cells that are harder to switch than others (due to fabrication and material nonuniformity) will fail to write consistently.
Accordingly, what is needed is a system and method for providing a magnetic memory element which can be used in a memory array of high density, low power consumption, low cross talk, and high reliability, while providing sufficient read signal. The present invention addresses the need for such a magnetic memory element.
The present invention provides a method and system for providing a magnetic element capable of being written using spin-transfer effect and a magnetic memory using the magnetic element. The magnetic element includes a spin tunneling junction and a spin valve, preferably separated by a separation layer. The spin tunneling junction includes a first free layer, a barrier layer and a first pinned layer. The first pinned layer is ferromagnetic and has a first pinned layer magnetization that is pinned in a first direction. The first free layer is ferromagnetic and has a first free layer magnetization. The barrier layer resides between the first pinned layer and the first free layer, is an insulator, and has a thickness that allows tunneling through the barrier layer. The spin valve has a second pinned layer, a nonmagnetic spacer layer and a second free layer. The second pinned layer is ferromagnetic and has a second pinned layer magnetization that is pinned in a second direction. The second free layer is ferromagnetic and has a second free layer magnetization. The nonmagnetic spacer layer is conductive and resides between the second free layer and the second pinned layer. The separation layer resides between the first free layer of the spin tunneling junction and the second free layer of the spin valve. The separation layer is configured to allow the first free layer and the second free layer to be magnetostatically coupled. In addition, the magnetic element is configured to allow the second free layer magnetization to change direction due to spin transfer when a write current is passed through the magnetic element in a CPP direction. Furthermore, the magnetic element can be shaped such that the spin valve portion has a smaller lateral dimension than the spin tunneling junction portion.
According to the system and method disclosed herein, the present invention provides a magnetic element and a magnetic memory that is capable of being written using the more efficient and localized spin-transfer mechanism while requiring a reduced current for writing and providing a high readout signal. In addition, the free layers of the spin valve and spin tunneling junction can be separately tailored to improve the functions of the spin valve and spin tunneling junction, respectively. Moreover, in embodiments in which the spin valve portion has a smaller lateral dimension, it is possible to have a higher current density in the spin valve portion than in the spin tunneling portion. This feature allows a sufficiently high current density for writing using the spin transfer effect while reducing the possibility that the barrier layer(s) of the spin tunneling junction will undergo breakdown.